1. Field of the Invention
This invention relates to an image signal processing device for effecting an image processing operation for suppressing nonuniformity of illumination or the like.
2. Description of the Related Art
In a general CRT monitor, the dynamic range for display is approx. 46 dB. Therefore, even when an image having a wider dynamic range or an image having a large difference in brightness between bright and dark portions is displayed, the bright and dark portions of the image cannot be correctly displayed at the same time.
For example, in an image created by an electronic endoscope (which is hereinafter referred to as an electronic scope) for displaying an electrical image signal obtained by photoelectric conversion using a solid state imaging device such as a CCD on a CRT display monitor, significant nonuniformity of illumination may be caused by a difference in distance between a light source and objects to be illuminated since the light source is substantially a point light source and, in the same image, strong halation may be caused on an object which lies near the light source and an object which is far from the light source is dark and nothing can be observed.
As a means for solving the above problem, an image signal correction circuit using a 2-dimensional filter as shown in Japanese Patent Disclosure No. 62-132479 was proposed.
In the above image signal correction circuit, the influence of nonuniformity of illumination is alleviated by effecting a sequence of operations of subjecting a luminance signal of the image signal to the logarithmic compression so as to permit an image having a wide dynamic range to be displayed and suppressing the low frequency range of a space frequency by use of a 2-dimensional filter and then converting the same into an exponential characteristic. Suppression of the low frequency component by use of the 2-dimensional filter is significantly effective when variation in the amount of illumination light is smooth as in an endoscope.
However, in an ordinary image other than an image created by the endoscope, a large number of so-called edge portions caused by contour portions of an object or, when the illumination light of step form is used for example, will be present and, if such an image is subjected to the same process as the above-described process, the edge portion thereof is forcedly or excessively emphasized and the image cannot be observed in a natural form.